SAMM 368, a myeloma which secretes IgG2b and IgA paraproteins, ion assembles heavy and light polypeptide chains of both immunoglobulin (Ig) clases correctly without molecular mixing. The IgG2b paraprotein is secreted five times as rapidly as IgA. The latter is assembled in the cytoplasm at a high rate but is released (secreted) slowly. Immunologic and immunogenetic conditions which are required for differentiation old plasma cells making each of the classes of immunglobulin have been studied. Condition can be found which favor either IgM or IgG synthesis.